Easy to forget Python library

Other libraries, such as insect, deque and queue can refer to: https://blog.csdn.net/a40850273/article/details/95356001

cmp_to_key function under functools

Python 3 has cancelled the cmp parameter of the built-in functions sort and sorted functions, so Python 3 sorting can only accept functions with a single input variable as the key parameter. However, a function with two input parameters can be passed in as the key parameter through cmp_to_key under functools. A simple example is given below.

from functools import cmp_to_key 
L=[9,2,23,1,2]
 
sorted(L,key=cmp_to_key(lambda x,y:y-x))
输出：
[23, 9, 2, 2, 1]
 
 
sorted(L,key=cmp_to_key(lambda x,y:x-y))
输出：
[1, 2, 2, 9, 23]

It should be noted that the function passed in cmp_to_key needs to return positive, negative and zero to express the relationship between the two numbers. If it is understood that two parameters are passed in in order, then returning a positive number means that the order of the two numbers needs to be changed, and vice versa.

heapq heap

Heapq implements the classic heap and provides the following methods:

heapq.heappush(heap, item) adds item to heap (heap is a list)
heapq.heappop(heap) pops the top element of the heap, and returns the top of the heap
heapq.heappushpop(heap, item) first add the item to the heap, then pop it, much faster than heappush() and heapop()
heapq.heapreplace(heap, item) pop first, and then add item to the heap, which is much faster than heapp() and then heappush()
heapq.heapify(x) Heap adjustment of list x, the default is small top heap
heapq.merge(*iterables) merges multiple lists and adjusts the heap, and returns an iterator of the merged list
heapq.nlargest(n, iterable, key=None) returns the largest n elements (Top-K problem)
heapq.nsmallest(n, iterable, key=None) returns the smallest n elements (Top-K problem)

import heapq
import random
 
# Top-K
mylist = list(random.sample(range(100), 10))
k = 3
largest = heapq.nlargest(k, mylist)
smallest = heapq.nsmallest(k, mylist)
print('original list is', mylist)
print('largest-'+str(k), '  is ', largest)
print('smallest-'+str(k), ' is ', smallest)
 
# heapify
print('original list is', mylist)
heapq.heapify(mylist)
print('heapify  list is', mylist)
 
# heappush & heappop
heapq.heappush(mylist, 105)
print('pushed heap is', mylist)
heapq.heappop(mylist)
print('popped heap is', mylist)
 
# heappushpop & heapreplace
heapq.heappushpop(mylist, 130)    # heappush -> heappop
print('heappushpop', mylist)
heapq.heapreplace(mylist, 2)    # heappop -> heappush
print('heapreplace', mylist)
'''
original list is [99, 44, 51, 45, 24, 82, 70, 97, 83, 3]
largest-3   is  [99, 97, 83]
smallest-3  is  [3, 24, 44]
original list is [99, 44, 51, 45, 24, 82, 70, 97, 83, 3]
heapify  list is [3, 24, 51, 45, 44, 82, 70, 97, 83, 99]
pushed heap is [3, 24, 51, 45, 44, 82, 70, 97, 83, 99, 105]
popped heap is [24, 44, 51, 45, 99, 82, 70, 97, 83, 105]
heappushpop [44, 45, 51, 83, 99, 82, 70, 97, 130, 105]
heapreplace [2, 45, 51, 83, 99, 82, 70, 97, 130, 105]
'''